In the technical field of the mobile communication, research and development have been actively carried out for a next mobile communication system after the conventional third-generation mobile communication system. The frequency band to be allocated to a future mobile communication system such as the forth-generation mobile communication system is expected to become wider due to the current demand for fast data transmission. In some cases, the allocated band may become so wide that the bands of the existing system are included in a part of the newly allocated frequency bands. In this case, the current system may be changed to the new system through a stage where the current and new systems exist together.
FIG. 1 illustrates examples how bands are allocated. In the “initial” stage, the bands of the current system are allocated on the left side (lower-frequency side). In this case, this system uses the Frequency Division Duplexing (FDD) scheme, and different frequencies are separately used between uplink and downlink transmissions. In next stage, additional bands are provided on the right side (higher-frequency side) for a new system. In the stage “After additional allocation” in FIG. 1, while the current system is used without changing the bands, the use of the new system is ready to be started. Then, eventually, as shown in the stage “Ideal allocation”, the service of the new system can be provided using a wider range of the frequency bands. However, during the progress of the stages of FIG. 1, namely, in the stage “After additional allocation”, there are plural guard intervals between an uplink band and a downlink band. This may not be preferable from the viewpoint of effective use of frequencies.
On the other hand, the guard interval or the guard band is provided between different system bands or for preparing for the switching the transmission direction between uplink and downlink. In a mobile radio communication system, not only the FDD scheme but also the Time Division Duplexing (TDD) scheme may be used. As described below, a relatively wider guard band is to be provided “between uplink band and downlink band in FDD”, “between band in FDD and band in TDD”, and “between bands using TDD” rather than “between uplink bands in FDD” and “between downlink bands in FDD”.
FIG. 2 illustrates a case where the frequency bands adjacent to each other (adjacent frequency bands) are used as downlink bands. In this case, each terminal receives a desired wave from the base station having a cell in which the terminal is located. Further the terminal also receives an interference wave from another base station. Generally, as long as a terminal of system X receives the desired wave from the most appropriate base station, the influence of the interference wave from the base station Y of system Y the terminal of system X is relatively small. Similarly, in a case where the frequency bands adjacent to each other (adjacent frequency bands) are used as uplink bands, the base station of one system may not receive significant interference from the base station of another system. Therefore, when the adjacent frequency bands are used as the same uplink bands or the same downlink bands, the guard band to be provided between the adjacent frequency bands can be relatively narrower.
FIG. 3 illustrates a case where one of the adjacent frequency bands is used as downlink band and the other one of the adjacent frequency bands is used as uplink band (i.e., the downlink band and the uplink band are adjacent to each other). In this case, while the terminal of the system X transmits data, the base station of the other system Y also transmits data. As a result, the waves from the base stations interfere with each other. In this case, the base station may typically receive greater interference from the other base station than interference from the terminal. This is because when compared with the interference from the terminal to the base station, the interference between the base stations is greater because the base station has higher receiving sensitivity and higher antenna height and may receive the transmitted interference wave from further location.
FIG. 4 more specifically illustrates a case where one of the adjacent frequency bands is used as downlink band and the other one of the adjacent frequency bands is used as uplink band. As illustrated in FIG. 4, the base station C receives a desired wave (uplink signal) from the terminal C and further receives an interference wave from other base station A. In this case, though it is not intent to limit the corresponding relationship, for example, the base station C, the base station X, and the terminal C in FIG. 4 correspond to the base station X, the base station Y, and the terminal X, respectively, in FIG. 3. As described above, the source of the interference in uplink from other base station is the base station. Therefore, when compared with a case of the interference from the terminal, the transmission power and the antenna gain become greater. Further, the influence of the interference between the bands may become greater due to, for example, the influence of the propagation environment with good view.
Also, FIG. 5 more specifically illustrates a case where one of the adjacent frequency bands is used as downlink band and the other one of the adjacent frequency bands is used as uplink band. However, the directions of the uplink and downlink in FIG. 5 are different from those in FIG. 4. As illustrated in FIG. 5, the terminal A receives a desired wave (downlink signal) from the base station A, and further receives an interference wave from other terminal C. In this case, though it is not intent to limit the corresponding relationship, for example, the terminal A, the base station A, and the terminal C in FIG. 5 correspond to the terminal Y, the base station Y, and the base station X, respectively, in FIG. 3. In this case, the source of the interference in downlink from terminal is the terminal. Therefore, when compared with a case of the interference from the base station, the transmission power and the antenna gain become smaller. However, it should be noted that there is possibility that the terminals approach each other within a short distance. In this case, the interference between the bands may be increased. For example, when the terminals are both located at the edge of the cell, the terminals may approach each other within a short distance.
Therefore, the guard band between an uplink band and a downlink band adjacent to the uplink band is required to be wider than the guard band between the uplink bands adjacent to each other and the guard band between the downlink bands adjacent to each other.
Further, in addition to the above, in the FDD scheme, the base station and terminals may receive data while transmitting data. Because of this feature, it may be required that the frequency band for the data transmission is sufficiently separated from the frequency band for the data reception.
To solve the problem described above, a method is disclosed of using software-defined radio technique and the Cognition supporting Pilot Channel (CPC) as illustrated in FIG. 6 (see Non-Patent Document 1).
Non-Patent Document 1: An alternative concept to scanning process for cognitive radio systems: technical and regulatory issues IEEE